The application claims priority to U.S. Provisional Application No. 60/537,527 which was filed on Jan. 20, 2004, the disclosure of which is incorporated in its entirety herein by reference.
This application relates to an indoor central heating, ventilation, and air conditioning (HVAC) system wherein various units report environmental characteristics to a central control for evaluation in relation to a user input. The central control controls an indoor relative humidity to prevent condensation inside a “building envelope.” A building envelope is defined to include all building exterior walls, i.e., walls having a side exposed to the outside elements and the roof.
Relative humidity is defined as the ratio of the actual amount of moisture in the air to the maximum moisture capacity at a given air temperature. It is known that as temperature increases, the capacity of the air to hold moisture in the form of water vapor also increases. Conversely, as temperature decreases, the capacity of the air to hold moisture decreases and any excess moisture condenses as water on surfaces in contact with the air.
Therefore, during the winter months, the cold outdoor air has a relatively low moisture content, however, the air inside building structures is typically heated. Depending on the construction quality of a particular building, some of the cold dry outside air infiltrates into the warm indoor space and is subsequently heated to the indoor temperature. This phenomenon effectively reduces the indoor relative humidity and the indoor air becomes very dry.
To address this winter dryness, humidifiers are often employed as part of the central heating system. Humidifiers introduce moisture into the heated air, increasing indoor relative humidity. Humidifiers are typically controlled by devices known as humidistats. Humidistats sense an actual indoor relative humidity and allow a homeowner to set a desired indoor relative humidity level. When the indoor relative humidity falls below the desired level, the humidistat activates the humidifier to add moisture to the air. Once the desired indoor humidity is achieved, the humidistat deactivates the humidifier.
Buildings typically have thermally insulated walls and attics to minimize heat loss and reduce cold air infiltration. However, portions of the building envelope, such as windows, may be less insulated than others, and their interior surfaces may get colder. If the outdoor temperature is low enough and the indoor humidity high enough, moisture may condense on these less insulated interior surfaces, which is undesirable. Conversely, some buildings in colder climates are built to be extremely “tight” allowing minimal outdoor air infiltration levels. Without the natural drying due to outside air infiltration, internal moisture generated by the occupants and their activities allows the indoor relative humidity to reach high levels resulting in condensation even in the winter months.
To address the concern of high indoor relative humidity, devices known as ventilators are often employed. Once the indoor relative humidity exceeds the desired level, the ventilator is activated to bring a controlled amount of outside dry air into the building envelope to decrease the indoor relative humidity. Ventilators typically are controlled by a second humidistat, separate from and in addition to the humidistat that controls the humidifier.
In general, the colder it is outside, the lower the indoor relative humidity has to be to avoid condensation. Therefore, occupants typically notice condensation when the weather turns cold and respond by lowering the humidistat setting. However, as weather patterns change, frequent manual adjustment is often required. To date there has been no way for the occupant to know exactly how much to adjust the humidity setting. This continual trial and error process results in the indoor relative humidity level either being too high or too low in comparison with the ideal indoor humidity level.
Therefore, controlling indoor relative humidity to a fixed relative humidity level, as with a simple humidistat, is undesirable.
While systems have been proposed to perform detailed calculations of a maximum relative humidity level, the known proposed are quite complex. As such, it is desirable to have an HVAC system that simply, but accurately, determines the maximum allowable indoor relative humidity to prevent condensation inside a building envelope based upon indoor and outdoor temperatures.